1. Field of the Invention
One embodiment of the present invention relates to a phosphorescent organometallic iridium complex that is capable of converting a triplet excited state into luminescence. In addition, one embodiment of the present invention relates to a light-emitting element, a light-emitting device, an electronic device, and a lighting device each including the phosphorescent organometallic iridium complex.
2. Description of the Related Art
A light-emitting element having a structure in which a light-emitting layer containing an organic compound that is a light-emitting substance is provided between a pair of electrodes has attracted attention as a next-generation flat panel display element in terms of characteristics such as being thin and light in weight, high speed response, and direct current low voltage driving. Further, a display including this light-emitting element is superior in contrast, image quality, and wide viewing angle.
Some of organic compounds which can be used for a light-emitting layer are capable of emitting phosphorescence from an excited state. Phosphorescence refers to luminescence generated by transition between energies of different multiplicity. In an ordinary organic compound, phosphorescence refers to luminescence that is generated at the time of relax from a triplet excited state to a singlet ground state (in contrast, fluorescence refers to luminescence that is generated at the time of relax from a singlet excited state to a singlet ground state). When such a compound capable of emitting phosphorescence, i.e., converting a triplet excited state into luminescence (hereinafter referred to as phosphorescent compound), is used as a light-emitting substance in a light-emitting layer, internal quantum efficiency can be increased; thus, a highly efficient light-emitting element can be obtained.
A phosphorescent organometallic iridium complex that contains iridium or the like as a central metal is particularly attracting attention as a phosphorescent compound because of its high phosphorescence quantum yield (refer to Patent Document 1, Patent Document 2, and Patent Document 3, for example).